Groups

I have two pearl guineas and two royal purple guineas. Starting a few months ago, (we got them in late June when they were a day old) there was one purple guinea who always was excluded from the group of the other three guineas. I was thinking that it was just that guineas personality and sometimes they do stay together. However for the most part the three guineas stick together while the other one does whatever he/she wants. So is this the guinea's personality or is it because the guinea is a female or something completely different?